U.S. patent number 4,727,293 [Application Number 06/849,052] was granted by the patent office on 1988-02-23 for plasma generating apparatus using magnets and method.
This patent grant is currently assigned to Board of Trustees operating Michigan State University. Invention is credited to Jes Asmussen, Mahmoud Dahimene, Donnie K. Reinhard.
United States Patent |
4,727,293 |
Asmussen , et al. |
February 23, 1988 |
Plasma generating apparatus using magnets and method
Abstract
An improved ion generating apparatus for producing a plasma disk
using magnets 34 and 35 around a region 16 in a chamber 15
positioned in a microwave cavity is described. The apparatus is
particularly operated at a microwave frequency such that electron
cyclotron resonance is produced in the plasma to create an
accelerating surface for the electrons around and inside of the
plasma. The apparatus can be operated to treat an article 100 in
the plasma or a holder 39, with a grid 51 to withdraw particles or
with a magnets 47 around an opening 48 forming a nozzle which with
electron cyclotron resonance produces a neutral beam of charged
particles. The apparatus is particularly useful as a plasma source
especially for oxidation, etching and deposition.
Inventors: |
Asmussen; Jes (Okemos, MI),
Reinhard; Donnie K. (East Lansing, MI), Dahimene;
Mahmoud (East Lansing, MI) |
Assignee: |
Board of Trustees operating
Michigan State University (East Lansing, MI)
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Family
ID: |
27093702 |
Appl.
No.: |
06/849,052 |
Filed: |
April 7, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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641190 |
Aug 16, 1984 |
4585668 |
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468897 |
Feb 23, 1983 |
4507588 |
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Current U.S.
Class: |
315/111.41;
118/723MA; 118/723MR; 204/298.37; 204/298.38; 313/231.31;
313/362.1; 315/111.71; 315/111.81; 427/570; 427/575 |
Current CPC
Class: |
H01J
27/16 (20130101); H01J 37/32192 (20130101); H01J
37/32678 (20130101); H01J 37/32284 (20130101); H01J
37/32256 (20130101) |
Current International
Class: |
H01J
27/16 (20060101); H01J 37/32 (20060101); H01J
007/24 (); H05B 031/26 () |
Field of
Search: |
;315/111.41,111.71,111.81,111.91,111.21,111.31,39
;313/231.3,360.1,362.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0054621 |
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Jun 1982 |
|
EP |
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0184812 |
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Jun 1986 |
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EP |
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Primary Examiner: Chatmon; Saxfield
Attorney, Agent or Firm: McLeod; Ian C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No.
641,190, filed Aug. 16, 1984, now U.S. Pat. No. 4,585,668, which is
a continuation-in-part of Ser. No. 468,897 filed Feb. 23, 1983, now
U.S. Pat. No. 4,507,588.
Claims
We claim:
1. A plasma generating apparatus including a plasma source
employing a radio frequency, including UHF or microwave, wave
coupler of a non-magnetic metal in the shape of a hollow cavity
which can be excited in one or more TE or TM modes of resonance,
including an electrically insulated chamber having a central
longitudinal axis and mounted in the coupler, including a gas
supply means for providing a gas which is ionized to form the
plasma in the chamber, including a moveable plate means of a
non-magnetic metal in the cavity mounted perpendicular to the axis
of the chamber and moveable towards and away from the chamber as a
sliding short, including a moveable probe connected to and
extending inside the coupler for coupling the radio frequency waves
to the coupler, wherein movement of the moveable plate means and
the probe in the coupler achieves the selected TM or TE mode of
resonance of the radio frequency wave in the coupler and varies the
resonance of the mode and wherein the radio frequency wave applied
to the coupler creates and maintains the plasma at reduced
pressures in the shape of an elongate plasma disk perpendicular to
and surrounding the central longitudinal axis in the chamber, the
improvement which comprises:
(a) a plurality of first magnets mounted on the apparatus around
the longitudinal axis of the chamber on a ring of high permeability
magnetic material so as to create magnetic cusps in the chamber
which aid in confining the plasma in the chamber; and
(b) second magnets mounted on the apparatus on a sheet of high
permeability magnetic material so as to provide magnetic cusps
inside the chamber which aid in confining the plasma in the
chamber.
2. A plasma generating apparatus which comprises:
(a) a plasma source employing a radio frequency, including UHF or
microwave, wave coupler which is metallic and non-magnetic and in
the shape of a hollow cavity which can be excited in one or more TE
or TM modes of resonance;
(b) an electrically insulated chamber having a central longitudinal
axis and mounted in the coupler;
(c) gas supply means for providing a gas which is ionized to form
the plasma in the insulated chamber;
(d) a movable probe connected to and extending inside the coupler
for coupling the radio frequency waves to the coupler;
(e) a plurality of first magnets mounted around the longitudinal
axis of the chamber on a ring of high permeability magnetic
material so as to create magnetic cusps in the chamber which aid in
confining the plasma in the chamber;
(f) movable plate means as a sliding short in the cavity which is
non-magnetic and metallic mounted perpendicular to the axis and
movable towards and away from the chamber;
(g) second magnets mounted on a sheet of high permeability magnetic
material and adjacent to the plate means so as to provide magnetic
cusps inside the chamber which aid in confining the plasma in the
chamber; and
wherein movement of the plate means and the probe in the coupler
achieves the selected TE or TM mode of resonance of the radio
frequency wave in the coupler and varies the resonance of the mode
and wherein the radio frequency wave applied to the coupler creates
and maintains the plasma at reduced pressures in the shape of an
elongate plasma disk perpendicular to and surrounding the central
longitudinal axis in the chamber which is confined in the chamber
by the magnetic cusps.
3. The apparatus of claim 2 wherein an ion attracting means is
mounted adjacent an opening in the chamber to remove ions from the
chamber.
4. The apparatus of claim 2 wherein a support means is mounted
adjacent to an opening in the chamber for providing a surface to be
treated in the plasma.
5. The apparatus of claim 2 wherein some of the first magnets
mounted around the cavity adjacent the chamber form connecting
magnetic cusps with the second magnets on the moveable metal
plate.
6. The apparatus of claim 5 wherein the first magnets are mounted
inside the chamber and are covered with a thin non-magnetic
material so as to seal the magnets inside the chamber from charged
and energetic species of the plasma.
7. The apparatus of claim 2 wherein the second magnets are mounted
on the moveable metal plate adjacent the chamber.
8. The apparatus of claim 2 wherein the second magnets are mounted
on or adjacent to the moveable metal plate with a thin section of
the metal plate between the magnets and the chamber.
9. The apparatus of claim 2 wherein the high magnetic permeable
material is iron.
10. The apparatus of claim 2 wherein the second magnets and sheet
essentially cover the metal plate.
11. The apparatus of claim 10 wherein the chamber has an opening
and wherein additional magnets are mounted around the opening to
provide a magnetic nozzle from the chamber.
12. The apparatus of claim 1 wherein the first and second magnets
are permanent rare earth magnets having a field strength of between
about 0.01 and 0.5 Tesla such that electron cyclotron resonance can
be achieved in the chamber.
13. The apparatus of claim 1 wherein the magnets are electromagnets
which provide a variable field strength and thus produces variable
electron cyclotron resonance zones in the chamber.
14. A method for forming a plasma which comprises:
(a) providing plasma generating apparatus including a plasma source
employing a radio frequency, including UHF or microwave, wave
coupler of a non-magnetic metal in the shape of a hollow cavity
which can be excited in one or more TE or TM modes of resonance,
including an electrically insulated chamber having a central
longitudinal axis and mounted in the coupler, including a gas
supply means for providing a gas which is ionized to form the
plasma in the chamber, including a movable plate means of a
non-magnetic metal in the cavity mounted perpendicular to the axis
of the chamber and moveable towards and away from the chamber as a
sliding short, including a moveable probe connected to and
extending inside the coupler for coupling the radio frequency waves
to the coupler, wherein movement of the moveable plate means and
the probe in the coupler achieves the selected TM or TE mode of
resonance of the radio frequency wave in the coupler and varies the
resonance of the mode and wherein the radio frequency wave applied
to the coupler creates and maintains the plasma at reduced
pressures in the shape of an elongate plasma disk perpendicular to
and surrounding the central longitudinal axis in the chamber, the
improvement which comprises: a plurality of first magnets mounted
on the apparatus around the longitudinal axis of the chamber on a
ring of high permeability magnetic material so a to create magnetic
cusps in the chamber which aid in confining the plasma in the
chamber; and second magnets mounted on the apparatus on a sheet of
high permeability magnetic material so as to provide magnetic cusps
inside the chamber which aid in confining the plasma in the
chamber; and
(b) forming the plasma disk in the chamber confined by the magnetic
cusps.
15. The method of claim 14 wherein the magnets are permanent rare
earth magnets having a field strength between about 0.01 and 0.5
Tesla and wherein the microwave frequency is between about 400
MegaHertz and 10 GigaHertz.
16. The method of claim 15 wherein the apparatus is operated at
electron cyclotron resonance in the chamber.
17. The method of claim 15 wherein the field strength is about 875
gauss and the frequency is about 2.45 GigaHertz.
18. The method of claim 15 wherein the electron cyclotron resonance
creates an accelerating electron cyclotron resonance surface
entirely inside the chamber.
19. The method of claim 18 wherein the electron cyclotron resonance
surface is positioned in the chamber by varying the strength and
position of the magnets relative to the chamber.
20. The method of claim 16 wherein the electron cyclotron resonance
facilitates generation of the plasma in the chamber at low
pressures of less than about 10 microns in the chamber.
21. The method of claim 16 wherein the plasma generated in the
chamber produce charged species having enhanced properties as a
result of the electron cyclotron resonance.
22. The method of claim 14 wherein the magnets are electromagnets
which provide a variable field strength.
23. A method for forming a plasma which comprises:
(a) providing a plasma generating apparatus which comprises a
plasma source employing a radio frequency, including UHF or
microwave, wave coupler which is metallic and non-magnetic and in
the shape of a hollow cavity and which is excited in one or more TE
or TM modes of resonance; an electrically insulated chamber having
a central longitudinal axis and mounted in the coupler; gas supply
means for providing a gas which is ionized to form the plasma in
the insulated chamber; a plurality of first magnets mounted around
the longitudinal axis of the chamber on a ring of high permeability
magnetic material so as to create magnetic cusps in the chamber
which aid in confining the plasma in the chamber; movable plate
means in the cavity which is non-magnetic and metallic mounted
perpendicular to the axis and movable towards and away from the
chamber; second magnets mounted on a sheet of high permeability
magnetic material to provide magnetic cusps inside the chamber
which aid in confining the plasma in the chamber; and a movable
probe connected to and extending inside the coupler for coupling
the radio frequency waves to the coupler, wherein movement of the
plate means and the probe in the coupler achieves the selected TE
or TM mode of resonance of the radio frequency wave in the coupler
and varies the resonance of the mode, wherein the radio frequency
wave applied to the coupler creates and maintains the plasma at
reduced pressures in the shape of an elongate plasma disk
perpendicular to and surrounding the central longitudinal axis in
the chamber which is confined in the chamber by the magnetic cusps
and wherein the magnets have a field strength and position adjacent
to or around the chamber sufficient to create electron cyclotron
resonance in the chamber; and
(b) forming the plasma disk in the chamber confined by the magnetic
cusps.
24. The method of claim 23 wherein the apparatus has an ion
attracting means adjacent an opening in the chamber and ions are
removed from the plasma.
25. The method of claim 23 wherein the apparatus has a support
means adjacent an opening in the chamber and a surface to be
treated is provided in the plasma.
26. The method of claim 23 wherein plasma which is formed is used
for plasma processing of substrates.
27. The method of claim 23 wherein the plasma formed has multimode
excitation to provide a more uniform plasma discharge.
Description
BACKGROFUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an improved plasma generating
apparatus and method which uses magnets to establish a multicusp
static magnetic field around the plasma region which is inside an
insulated chamber. In particular, the present invention relates to
an apparatus and method which uses the magnets to aid in confining
the plasma in the chamber and also produces electron cyclotron
resonance (ECR) zones within the chamber which impart energy to the
electrons in the plasma.
(2) Prior Art
The present invention relates to an improvement on the plasma
generating apparatus described in U.S. Pat. No. 4,507,588 by some
of the inventors herein. This patent disclosed some confining
magnetic field configurations without detail as to the mounting of
the magnets. The radial positioning of the elongate magnets on the
sliding short was effective in confining the charged particles,
particularly the electrons; however, electron cyclotron resonance
(ECR) was not discussed.
OBJECTS
It is therefore an object of the present invention to provide a
plasma generating apparatus which includes improved means for
mounting of magnets for confining the plasma. Further it is an
object of the present invention to provide a plasma generating
apparatus which uses electron cyclotron resonance (ECR) to impart
energy to the plasma at very low pressures (.ltoreq.10 microns).
These and other objects will become increasingly apparent by
reference to the following description and the drawings.
IN THE DRAWINGS
FIG. 1 is a front partial cross-sectional view of the preferred
plasma generating apparatus particularly illustrating the
positioning of the magnets 34 and 35 in relation to a dish or
chamber 15 to provide magnetic cusps 16a inside the dish.
FIG. 2 is a plan cross-sectional view along line 2--2 of FIG.
1.
FIG. 3 is a front partial cross-sectional view of a modified plasma
generating apparatus of the present invention wherein additional
magnets 38 are provided around the dish 15.
FIG. 4 is a plan cross-sectional view along line 4--4 of FIG.
3.
FIG. 5 is a cross-sectional view of a sliding short 12 wherein the
magnets 35a are mounted on the short.
FIG. 6 is a front partial cross-sectional view of the sliding short
12b and cylinder 10b wherein the magnets 38a are mounted in the
cylinder 10b.
FIG. 7 is a front partial cross-sectional view of a modified plasma
generating apparatus wherein magnets 47 surround an opening 48 in
the plasma region 16d which acts as a magnetic nozzle directing
electrons and ions out of the plasma region.
FIG. 8 is a plan view along line 8--8 of FIG. 7.
FIG. 9 is a front cross-sectional view of a modified base 30b for a
plasma generating apparatus wherein shielded magnets 45a are
provided inside the plasma region 16e.
FIG. 10 is a plan cross-sectional view along line 10--10 of FIG.
9.
FIG. 11 is an equivalent electrical circuit diagram which
approximates the circuit elements of the plasma generating
apparatus which is presented for the purpose of describing the
operation of the apparatus.
GENERAL DESCRIPTION
The present invention relates to a plasma generating apparatus
including a plasma source employing a radio frequency, including
UHF or microwave, wave coupler of a non-magnetic metal in the shape
of a hollow cavity which can be excited in one or more TE or TM
modes of resonance, including an electrically insulated chamber
having a central longitudinal axis and mounted in the coupler,
including a gas supply means for providing a gas which is ionized
to form the plasma in the chamber, including a moveable plate means
of a non-magnetic metal in the cavity mounted perpendicular to the
axis of the chamber and moveable towards and away from the chamber
as a sliding short, including a moveable probe connected to and
extending inside the coupler for coupling the radio frequency waves
to the coupler, wherein movement of the moveable plate means and
the probe in the coupler achieves the selected TM or TE mode of
resonance of the radio frequency wave in the coupler and varies the
resonance of the mode and wherein the radio frequency wave applied
to the coupler creates and maintains the plasma at reduced
pressures in the shape of an elongate plasma disk perpendicular to
and surrounding the central longitudinal axis in the chamber, the
improvement which comprises: a plurality of first magnets mounted
on the apparatus around the longitudinal axis of the chamber on a
ring of high permeability magnetic material so as to create
magnetic cusps in the chamber which aid in confining the plasma in
the chamber; and second magnets mounted on a sheet of high
permeability magnetic material in the apparatus so as to provide
magnetic cusps inside the chamber which aid in confining the plasma
in the chamber.
Further the present invention relates to a plasma generating
apparatus which comprises: a plasma source employing a radio
frequency, including UHF or microwave, wave coupler which is
metallic and non-magnetic and in the shape of a hollow cavity which
can be excited in one or more TE or TM modes of resonance; an
electrically insulated chamber having a central longitudinal axis
and mounted in the coupler; gas supply means for providing a gas
which is ionized to form the plasma in the insulated chamber; a
movable probe connected to and extending inside the coupler for
coupling the radio frequency waves to the coupler; a plurality of
first magnets mounted around the longitudinal axis of the chamber
on a ring of high permeability magnetic material so as to create
magnetic cusps in the chamber which aid in confining the plasma in
the chamber; movable metal plate means as a sliding short in the
cavity which is non-magnetic mounted perpendicular to the axis and
movable towards and away from the chamber; second magnets mounted
on a sheet of high permeability magnetic material and on the plate
means so as to provide magnetic cusps inside the chamber which aid
in confining the plasma in the chamber; and wherein movement of the
plate means and the probe in the coupler achieves the selected TE
or TM mode of resonance of the radio frequency wave in the coupler
and varies the resonance of the mode and wherein the radio
frequency wave applied to the coupler creates and maintains the
plasma at reduced pressures in the shape of an elongate plasma disk
perpendicular to and surrounding the central longitudinal axis in
the chamber which is confined in the chamber by the magnetic
cusps.
Further still the present invention relates to a method for forming
a plasma which comprises: a plasma generating apparatus including a
plasma source employing a radio frequency, including UHF or
microwave, wave coupler of a non-magnetic metal in the shape of a
hollow cavity which can be excited in one or more TE or TM modes of
resonance, including an electrically insulated chamber having a
central longitudinal axis and mounted in the coupler, including a
gas supply means for providing a gas which is ionized to form the
plasma in the chamber, including a moveable plate means of a
non-magnetic metal in the cavity mounted perpendicular to the axis
of the chamber and moveable towards and away from the chamber as a
sliding short, including a moveable probe connected to and
extending inside the coupler for coupling the radio frequency waves
to the coupler, wherein movement of the moveable plate means and
the probe in the coupler achieves the selected TM or TE mode of
resonance of the radio frequency wave in the coupler and varies the
resonance of the mode and wherein the radio frequency wave applied
to the coupler creates and maintains the plasma at reduced
pressures in the shape of an elongate plasma disk perpendicular to
and surrounding the central longitudinal axis in the chamber, the
improvement which comprises: a plurality of first magnets mounted
on the apparatus around the longitudinal axis of the chamber on a
ring of high permeability magnetic material so as to create
magnetic cusps in the chamber which aid in confining the plasma in
the chamber; and second magnets mounted on a sheet of high
permeability magnetic material so as to provide magnetic cusps
inside the chamber which aid in confining the plasma in the
chamber; and forming the plasma disk in the chamber confined by the
magnetic cusp.
Preferably the present invention utilizes rare earth magnets with a
field strength between about 0.01 and 0.5 Tesla. Superconducting
magnets can also be used to produce even higher magnetic field
strengths. In this later case, the applied magnetic fields can be
varied (by varying coil currents) to provide an optimum magnetic
field geometry. The magnetic field strength is preferably:
.omega.=2.pi.f and f is the cavity excitation frequency. m.sub.e is
the mass of the electron and e is the magnitude of the charge of
the electron.
While the apparatus described in U.S. Pat. No. 4,507,588 works
without an applied static magnetic field, the addition of a
magnetic field is a variation of the technology that has advantages
for many potential applications. These advantages are: (1) a
reduction of charged particle diffusion losses resulting in higher
discharge efficiencies, (2) providing ECR zones in the discharge
volume thereby enhancing electromagnetic coupling to the discharge
at low pressures (<10 microns) and low input gas flow rates, (3)
control of discharge uniformity by spatially adjusting the ECR
zones, (4) creating a group of high energy electrons yielding
different plasma chemistry than microwave discharges without ECR
magnetic fields and (5) providing a method for discharge ignition
at low pressures, i.e., electron cyclotron resonance breakdown.
This is an important feature of the present invention which allows
easy starting of the plasma discharge at low pressures.
The design approach for the microwave plasma/ion sources described
here is similar to that described in earlier U.S. Pat. No.
4,507,588. A microwave discharge is created in a disk shaped
discharge plasma region which is separated from the applicator
(cavity) aperture (or antenna) by a quartz confining enclosure or
disk. The applicator is in the shape of a hollow, cylindrical
cavity which focuses and matches the microwave energy into the
plasma region utilizing single or controlled multimode
electromagnetic excitation and "internal cavity" matching. This
apparatus can be used as a broad-beam ion source or as a plasma
source for materials processing.
SPECIFIC DESCRIPTION
FIGS. 1 and 2 show the preferred improved plasma generating
apparatus of the present invention. The basic construction of the
apparatus without the magnet mountings shown is described in U.S.
Pat. No. 4,507,588. It will be appreciated that various non
magnetic materials can be used in the construction of the
apparatus, such as copper, brass, aluminum, silver, gold, platinum,
non-magnetic stainless steel and the like.
The apparatus includes copper or brass cylinder 10 forming the
microwave cavity 11 with a copper or brass sliding short 12 for
adjusting the length of the cavity 11. Silver plated copper brushes
13 electrically contact the cylinder 10. The brushes 13 are
provided entirely around the circumference of the sliding short 12;
however, in FIG. 2 only 4 are shown. Moveable excitation probe 14
provides impedence tuning of the microwave energy in the cavity 11.
The probe 14 is mounted in cavity 11 by brass or copper conduit 21.
Radial penetration of the probe 14 into the cavity 11 varies the
coupling to the plasma in the cavity 11. Sliding short 12 is moved
back and forth in cavity 11 to aid in tuning the microwave by rods
22 using conventional adjustment means (not shown) such as
described in U.S. Pat. No. 4,507,588.
A quartz dish or chamber 15 shaped like a petri dish or round
bottle bottom defines the plasma region 16 along with a stainless
steel base 30 and holder 39. The holder 39 can have an electrical
bias (not shown), which can be D.C. or R.F. to attract ions from
the plasma. Gas is fed by tube 19 to annular ring 18 and then flows
into the plasma region 16. Optionally a cooling line 42 is provided
which cools the base 30. The cylinder 10 slides onto the base 30
and is held in place on base 30 by copper or brass ring 10a secured
to the cylinder 10. Sliding silver plated copper brushes 32 mounted
on a brass ring 31 contact the cylinder 10 to provide good
electrical contact. The ring 10a is held in place on base 30 by
copper or brass bolts 33. This construction allows the base 30 and
dish 15 to be removed from the cylinder 10. The basic device
operates without magnets as described in U.S. Pat. No.
4,507,588.
In the improved plasma apparatus, the dish 15 and plasma region 16
are surrounded by magnets on three sides. In the preferred
embodiment, eight (8) or more equally spaced magnets 34 surround
the dish 15 around axis a--a. A second set of magnets 35 is mounted
on sliding short 12 by means of a thin aluminum cup 12a. The
combination of the magnets 34 and 35 provide interconnected
magnetic field cusps 16a in the plasma region 16 of the dish 15 as
shown in FIG. 1. The magnets 34 and 35 reduce particle diffusion
losses from region 16 inside the dish 15. The magnetic field
strength decreases as the longitudinal axis a--a and center of the
plasma region 16 is approached because of the positioning of the
magnets 34 and 35.
The magnets 34 are mounted on a high magnetic permeability (iron)
ring 37 around the ring 31 and held in place by magnetic
attraction. The iron ring 37 is secured to brass ring 31 such as by
soldering. The ring 37 partially surrounds the magnets 34 in an "L"
shape so that the magnetic cusps 16a extends into dish 16 and then
terminates at the bottom leg of the "L". The magnets 35 are held in
place on a high magnetic permeability (iron) circular plate 36. The
plate 36 is fastened to sliding short 12.
One end of the dish 15 adjacent the holder 39 is free of magnets.
In this region, grids (not shown) or an article 100 to be treated
is mounted on holder 39 as described in Ser. No. 641,190, filed
Aug. 16, 1984. Gases pass out the opening 41 in tube 40 from the
plasma region 16. The magnets 34 and 35 thus surround the dish 15.
The plasma region 16 is surrounded by the interconnected magnetic
cusps 16a.
FIGS. 3 and 4 show a variation of the device of FIGS. 1 and 2
wherein the dish 15a is taller along the axis a--a. In this
embodiment, additional magnets 38 are mounted on high magnetic
permeability (iron) ring 44 secured to cylinder 10. This allows the
magnetic cusps 16b to join together as shown by FIGS. 3 and 4 in a
manner similar to that shown in FIG. 1. The remaining construction
of FIGS. 3 and 4 is otherwise identical to that of FIGS. 1 and
2.
In the following description, letters beside the reference numbers
are used for elements functionally identical to those of FIGS. 1 to
4. Where the function is the same, they are not necessarily
redescribed.
FIGS. 5 and 7 show a modified brass or copper sliding short 12b
with a recess 12c which supports magnets 35a on a thin portion 12d.
The magnets 35a are positioned on a circular iron plate 36a. In
this modification, the thin portion 12d allows the magnetic cusps
16c to form inside the dish 15c. FIG. 6 shows a variation wherein
the magnets 38a are mounted in slots 10c in cylinder 10b with a
thin portion 10d which allows the magnetic cusps (not shown) to
penetrate the dish (not shown). High magnetic permeability ring 37a
holds the magnets 38a in place on the cylinder 10b. Thus there are
a number of ways of mounting the magnets 34, 35, 35a, 38 and 38a on
the cylinder 10, 10a or 10b and sliding short 12, 12a or 12b. The
magnets 35 do not have to be mounted on the cylinder 10 or short 12
and can be independently mounted in or outside of the cavity 11. So
long as the cavity is constructed of a nonmagnetic material, the
magnetic cusps will penetrate the dish 15, 15a or 15b confining the
plasma region 16 or 16d if they are sufficiently strong and
properly positioned.
FIGS. 7 and 8 also show the use of magnets 45 located in the plasma
region 16d inside the dish 15 and also show magnets 47 surrounding
an opening 48 in a non-magnetic plate 46. The magnets 47 are sealed
from the plasma by a covering (not shown) and are secured to an
iron ring 47a. In this configuration, the magnets 45 and 47 produce
a reduced magnetic field region in opening 48 allowing the high
energy electrons to pass through the opening 48. The reduced field
region in the opening 48 thus allows electrons to be accelerated
from the plasma 16d and the ions are pulled along by electrostatic
forces through the opening 48. This provides thrust such as for an
ion engine or it can be used for ion treatment.
FIGS. 9 and 10 show a dish 15d defining a plasma region 16e.
Magnets 45a are provided around and in the region 16e and are
encased in a thin stainless steel shield 50 to prevent exposure to
the plasma in the region 16e. A voltage biased grid 51 is mounted
on a ring insulator 52 to isolate the grid 51. The grid 51 attracts
ions from the plasma region 16e. The plasma apparatus of FIGS. 9
and 10 otherwise has the same construction as FIG. 1 and 2 where
the letters inside the numbers have been used to identify
functionally identical elements.
FIG. 11 shows the equivalent electrical circuit of the cavity 11
and plasma region 16 in dish 15. This is discussed more fully
hereinafter.
One end of the disk shaped discharge region 16 is free of magnets.
It is in this region where the grid 51 (FIG. 9) for ion extraction
or the processing of material 100 on plate 39 (FIG. 1) is located.
The type, positions and connections of the grids/or processing
plate are as described in U.S. Pat. No. 4,507,588 and application
Ser. No. 641,190. It is important to note however, that the static
magnetic field at the grid 51 or plate 39 locations is made very
low (<50 gauss to zero) by placing a lip on the ring 37. This
"shorts out" the magnetic field from the adjacent strong magnets
34. The array of alternating poles of magnets 35 also produce
little static magnetic field in the location of the grids 51 or
plate 39.
It is well known that at electron cyclotron resonance (ECR) energy
can be efficiently transferred from an electromagnetic field to
electrons. In the plasma region 16 this energy in turn is
transferred to the electron gas via electron-electron and electron
heavy molecule collisions or to the heavier ion and neutral gases
via electron-ion and electron-neutral collisions. ECR occurs when
the exciting radian frequency .omega. equals the cyclotron radian
frequency .omega..sub.b.
wherein
e is the magnitude of the charge of the electron
m.sub.e is the mass of the electron
B is the static magnetic field strength.
Expressed in terms of frequency
Thus, for an exciting frequency of 2.45.times.10.sup.9 Hz the
static magnetic field required for ECR is approximately 0.0875
Tesla or 875 gauss. It is also noted that the average cyclotron
radius r.sub.c for electrons is given by
where V.sub.t is the rms thermal velocity of the electron gas
(V.sub.t =3.89.times.10.sup.3 T.sub.e.sup.1/2 m/sec for electrons).
Thus, for plasmas where the electron temperature, T.sub.e, is
10.sup.5 .degree.K. or less the cyclotron radius is less than one
millimeter. Thus, electron acceleration takes place in a small,
thin layer (or volume) called an ECR surface, around the magnets 34
and 35. These ECR zones can be observed in the discharge as
intensely glowing thin discharge layers resulting from the gas
excitation by the accelerated electrons.
If the magnets 34 and 35 are strong enough, the ECR zone (or
surface) occurs within the discharge region 16. The dotted lines 53
and 53a in FIG. 1 and 3 show examples of such ECR zones (or
surfaces) located in the plasma region of the quartz dish 15. As
can be observed from these figures these zones 53 and 53a trace out
a three dimensional surface within the region 16. Whenever an
electron passes through this surface it experiences an energy gain
from the time varying electromagnetic field if the electric field
has a component perpendicular to the static magnetic field lines.
Thus, an electron will move through the discharge by reflecting
from magnetic cusp (16a or 16b) to magnetic cusp and gaining energy
each time it passes through an ECR surface. Most electrons then
give up energy via collisions throughout the discharge. The ECR
zone positions can be varied by increasing or decreasing the
strength of the magnets 34, 35, 36 and 38. Increasing the magnetic
field strengths moves the ECR surface away from the walls further
into the center of the discharge. Decreasing the magnet strengths
moves the surface toward the walls of the dish 15 or 15a. It is
often desirable to have the ECR surface located entirely within the
region 16 as shown in the Figures and not cutting through the
quartz dish 15 or 15a.
This method of electron gas heating has been employed in other ion
and plasma sources (R. Geller and F. Gugliermotte, U.S. Pat. No.
4,417,178; T. Consoli, L. Saint-Cloud, R. B. Clamart, R. Geller, A.
Bernard, U.S. Pat. No. 3,571,734; and R. Geller, IEEE Trans. Nucl.
Sci. NS-23, 904 (1976)). However, the present concept differs in
cavity tuning, excitation, and geometry, and is able to produce a
large, magnetic field free plasma surface in plasma region 16 for
ion extraction or plasma processing. Another important difference
is that the discharge region and cavity 11 are separate, allowing
each to be optimized individually. Thus, the optimization of the
discharge volume and shape, and discharge matching are more
independent. This produces a highly ionized discharge with
densities much greater than the critical density while using very
low incident power levels, typically not more than 100 to 200
watts.
Variations of the present technology are many. For example, the
cusp magnetic field strengths can be adjusted to provide: (1) ECR
surfaces surrounding the entire end and cylindrical side walls of
the dish 15 as shown in FIGS. 1 and 2, (2) an ECR surface in one
region of the discharge and just a confining magnetic fields in
other regions. For instance, ECR zones can be created in the top of
the discharge region by magnets 35 while the magnets 34 and 38
provide a confining field. This also can be reversed where magnets
35 produce the confining cusps fields 16b over the top of the
discharge and the ring magnets 34 create the ECR zones as well as
confining fields.
Other variations of this technology allow magnets 35a and 38a to be
placed outside the applicator electromagnetic excitation volume as
is shown in FIGS. 5 and 6. Also, the applicator sliding short 12
can be adjusted separately from the end plate magnets 35.
Another variation of this technology, shown in FIGS. 7 and 8,
encloses all sides of the disk shaped discharge region 16e with
magnetic cusps. As shown, the base 30a is also enclosed with an
array of permanent magnets 47 except for a central region where a
magnet is removed to form the opening 48 (where the magnetic field
is deliberately reduced). High energy electrons accelerated via ECR
in the discharge will escape through this low magnetic field region
of the opening 48 i.e., a magnetic nozzle is produced. The
electrons will in turn pull by electrostatic forces the heavier
lower energy ions along through the opening 48 of the nozzle
producing a neutral beam of charged particles. This neutral beam 49
can then be used for material processing, space engine
acceleration, and the like. An advantage of this configuration is
that the neutral, high density beam can be produced without the
need for the usual grid optics of FIG. 9.
An important feature of this ion and plasma source is its ability
to match (i.e., operate with zero or very little reflected power)
the incident microwave power into the low, variable pressure
(.sup..about. 10 micron) disk plasma region 16 for many different
discharge conditions. Variable cavity 11 length and variable
coupling probe 14 tuning allow the discharge to be matched over a
wide range of discharge pressure, input powers, gas flows and gas
mixtures, etc. This match is accomplished using single mode or
controlled mutlimode excitation and hence is accomplished without
altering either the plasma shape or the applied electromagnetic
field patterns and without losing microwave power in external
tuning stubs (not shown). Increases in input power increase the
electric and magnetic field strengths, however, the geometry of the
mode field patterns, remains approximately constant throughout the
tuning process keeping the geometry of the fields exciting the
plasma region 16, i.e., the electromagnetic focus, constant.
Preferably the microwave frequency is between about 400 MegaHertz
and 10 GigaHertz.
The input impedance of a microwave cavity is given by ##EQU1##
where P.sub.t is the total input power coupled into the cavity 11
(which includes metal wall losses as well as the power delivered to
the discharge), W.sub.m and W.sub.e are, respectively, the
time-averaged magnetic and electric energy stored in the cavity
fields and .vertline.I.sub.o .vertline. is the total input current
on the coupling probe. R.sub.in and jX.sub.in are the cavity 11
input resistance and reactance and represent the complex load
impedance as seen by the feed transmission line.
At least two independent adjustments are required to match this
load to a transmission line. One adjustment must cancel the load
reactance while the other must adjust the load resistance to the
characteristic impedance of the feed transmission system. In the
cavity 11 the continuously variable probe 14 and end plate 12
tuning provide these two required variations, and together with
single mode excitation are able to cancel the discharge reactance
and adjust the discharge resistance to equal the characteristic
impedance of the feed transmission line.
The internal cavity 11 matching technique employed in the
applicator can be understood with the aid of the equivalent circuit
shown in FIG. 11. This is a standard circuit representation for a
cavity 11 which is connected to a feed waveguide or transmission
line and is excited in the vicinity of a single mode resonance.
G.sub.c, L.sub.c and C.sub.c represent the conductance, inductance
and capacitance respectively of the excited mode near resonance and
the jX represents the reactive effect of the evanescent modes far
from resonance. The coupling probe 14 (or aperture) is represented
as the ideal transformer 60 of turns ratio M:L and coupling
reactance jX. Both circuit elements and the transformer 60 are
drawn with arrows to indicate their variability during the tuning
process. At resonance, the capacitive and inductive susceptance
cancel resulting in a pure conductive input admittance.
The discharge is ignited by first adjusting the probe 14 and cavity
11 length positions to excite a specific empty cavity 11 resonance
and to match the empty cavity 11 applicator to the input
transmission system. Microwave power is then applied, absorbed into
the cavity 11 without reflection and a discharge is ignited in
plasma region 16 even with low input powers of 10-20 W if the
pressure in the disk discharge zone is reduced to less than 10
Torr. The presence of the discharge then changes L.sub.c, G.sub.c,
and C.sub.c and adds an additional discharge conductance G.sub.L
and susceptance, jB.sub.L, to the circuit. That is, in the presence
of a discharge in plasma region 16, the equivalent circuit elements
become nonlinear functions of many experimental variables. These
include discharge gas mix and type, pressure and flow rate,
discharge geometry, absorbed microwave power (i.e., electric field
strength squared, E.sup.2) and discharge properties such as
electron density, and collision frequency. The nonlinear behavior
of the discharge in region 16 (and hence the behavior of the
equivalent circuit element) is exhibited as hysteresis in
experimental variables such as input power, tuning, and operating
pressure.
The discharge admittance shifts the resonance, unmatching the
plasma loaded cavity 11 from the feed transmission line. If the
cavity 11 length and coupling probe 12 remain fixed, further
increases in incident power result in only a slight increase in
absorbed power and a small change in discharge admittance resulting
in mismatching and untuning the cavity 11 from resonance. Thus, the
presence of the discharge allows only a small portion of the
additional incident power into the cavity 11 causing a large
increase in reflected power. This limited variation in discharge
properties is a fundamental problem associated with sustaining
microwave discharges in fixed size and fixed coupling cavities (S.
L. Halverson and A. J. Hatch, Appl. Phys. Lett. 14, 79 (1969)).
Discharges in these cavities 11 can be only maintained over a very
narrow range of discharge loads (discharge densities, volumes,
pressures, flow rates, etc.) and thus, these cavity 11 applicators
often operate with large reflected powers.
The variable internal cavity 11 matching employed in the apparatus
of the present invention provides the variable impedance
transformation that allows the discharge to be matched over a wide
range of discharge loads. For a given incident power, gas type, gas
flow rate, and discharge pressure, i.e., for a given operating
condition, the length and probe tuning are varied iteratively until
reflected power is reduced to zero. This tuning together with
variation of the incident microwave power "pulls" the discharge
properties along a discharge "loss line" similar to that described
elsewhere for cylindrical discharges (R. M. Fredericks and J.
Asmussen, J. Appl. Phys. 42, 3647 (1971); J. Asmussen, R.
Mallavarpu, J. R. Hamann, and H. R. Park, Proc. IEE 62, 109 (1974);
and R. Mallavarpu, M. C. Hawley and J. Asmussen, IEEE Trans. Plasma
Sci. PS-6, 341 (1978)).
Typical tuning distances are of the order of several millimeters to
one-half centimeter and thus, the tuning process can be quickly
performed either manually or with small motors (not shown). The
tuning can also be utilized as a simple discharge power control
technique. The motors, placed on the sliding short 12 and probe 14
allow length and probe tuning to be performed while the cavity 11
is held at a high potential.
The excitation of a discharge in the presence of a static magnetic
field 34, 35 and 38 is more difficult to understand and model than
the simple case of a discharge without a static magnetic field. It
is well known that linear, cold plasma theory predicts that the
equivalent dielectric constant of a gyrotropic plasma (a plasma
immersed in a static magnetic field) is a complex dyadic (a tensor
of order 2). In such a gyrotropic plasma energy is not only
absorbed from an electromagnetic field by the electron gas via
electron heavy particle collisions but the discharge electrons also
absorb energy at the electron cyclotron resonance. In this later
case, energy is efficiently absorbed in the cyclotron resonant
zones if the applied electric field has a component perpendicular
to the static magnetic field. Thus, in the cavity 11 applicator the
applied electric field must have an electric field component
tangential to a cyclotron resonant surface. There are many empty
cavity 11 modes that can provide such a field over part or most of
the ECR surfaces.
Once the discharge is ignited, the cavity 11 mode field patterns
change from the empty cavity 11 field geometries due to the
presence of the plasma. These changes are difficult to model and
calculate especially in this ion source since the static magnetic
field varies considerably (from cusp 16a to cusp) over a standing
electromagnetic wavelength. However, the probe 14 and sliding short
12 tuning allow optimum, well-matched, discharge conditions to be
found empirically. Since the ECR zones makes the plasma very lossy
(easily absorbs microwave energy) at low pressure, the operating
cavity Q is lower and the cavity 11 tuning becomes simpler.
Excitation of the cavity 11 applicator system with more than one
mode is possible if the cavity 11 dimensions and the discharge
properties and size are adjusted for a multimode interaction. An
obvious example of multimode operation is the excitation of two,
empty cavity 11 degenerate modes such as the TE.sub.011 and
TM.sub.111 modes. However, the presence of the discharge in the
plasma region 16 will shift the resonance of these two modes
differently removing the degeneracy. Thus, multimode excitation
with a discharge must take into account the properties of the
discharge. These multimode intersections have been investigated (J.
Asmussen, R. Mallavarpu, J. R. Hamann, and H. R. Park, Proc. IEE
62, 109 (1974); and R. Mallavarpu, M. C. Hawley and J. Asmussen,
IEEE Trans. Plasma Sci. PS-6, 341 (1978) for a cylindrical cavity
11 applicator and cylindrical discharge in plasma region 16. In
practice, "closely spaced" but non-intersecting modes can also
interact, especially if each has a low Q (or large excitation
bandwidth).
The dashed equivalent circuit of FIG. 11 indicates the
modifications of the circuit required to include an extra mode.
Multimode excitation may result in practical solutions to discharge
excitation such as discharge uniformity. For example, if two
TE.sub.111 modes are excited by the same microwave source and are
both separated by 90.degree. in time and space, a circulating
polarized electromagnetic mode can be excited in the cavity 11. The
rotating field due to this mode then will produce a more uniform
discharge.
The present invention is particularly useful for plasma processing
of various substrates. This especially includes plasma oxidation,
etching and deposition.
As can be seen from the foregoing description, the present
invention provides unique plasma generating apparatus. The
especially mounted magnets and magnetic cusps produced thereby
confine the plasma and preferably produce ECR. Numerous variations
will occur to those skilled in the art and it is intended that the
present invention be limited only by the hereinafter appended
claims.
* * * * *